Geometric Modeling Notes on Curve and Surface Continuity Parts of Mortenson, Farin, Angel, Hill and others

From Previous Lectures

Continuity at Join Points (from Lecture 2) Discontinuous: physical separation Parametric Continuity Positional ( C 0 ): no physical separation C 1 : C 0 and matching first derivatives C 2 : C 1 and matching second derivatives Geometric Continuity Positional ( G 0 ) = C 0 Tangential ( G 1 ) : G 0 and tangents are proportional, point in same direction, but magnitudes may differ Curvature ( G 2 ) : G 1 and tangent lengths are the same and rate of length change is the same source: Mortenson, Angel (Ch 9), Wiki

Continuity at Join Points Hermite curves provide C 1 continuity at curve segment join points. –matching parametric 1 st derivatives Bezier curves provide C 0 continuity at curve segment join points. –Can provide G 1 continuity given collinearity of some control points (see next slide) Cubic B-splines can provide C 2 continuity at curve segment join points. –matching parametric 2 nd derivatives source: Mortenson, Angel (Ch 9), Wiki

Composite Bezier Curves (from Lecture 3) Evaluate at u =0 and u =1 to show tangents related to first and last control polygon line segment. Joining adjacent curve segments is an alternative to degree elevation. source: Mortenson Collinearity of cubic Bezier control points produces G 1 continuity at join point: For G 2 continuity at join point in cubic case, 5 vertices must be coplanar. (this needs further explanation – see later slide)

Composite Bezier Surface (from Lecture 5) Bezier surface patches can provide G 1 continuity at patch boundary curves. For common boundary curve defined by control points p 14, p 24, p 34, p 44, need collinearity of: Two adjacent patches are C r across their common boundary iff all rows of control net vertices are interpretable as polygons of C r piecewise Bezier curves. source: Mortenson, Farin Cubic B-splines can provide C 2 continuity at surface patch boundary curves.

Supplemental Material

Continuity within a (Single) Curve Segment (from Prior Lecture) Parametric C k Continuity: –Refers to the parametric curve representation and parametric derivatives –Smoothness of motion along the parametric curve –“A curve P ( t ) has k th-order parametric continuity everywhere in the t -interval [ a, b ] if all derivatives of the curve, up to the k th, exist and are continuous at all points inside [ a, b ].” –A curve with continuous parametric velocity and acceleration has 2 nd -order parametric continuity. source: Hill, Ch 10 apply product rule 1 st derivatives of parametric expression are continuous, so spiral has 1 st -order ( C 1 ) parametric continuity. Note that C k continuity implies C i continuity for i < k. Example

Continuity within a (Single) Curve Segment (continued) Geometric G k Continuity in interval [ a, b ] (assume P is curve): –“Geometric continuity requires that various derivative vectors have a continuous direction even though they might have discontinuity in speed.” –G 0 = C 0 –G 1 : P’ ( c- ) = k P’ ( c+ ) for some constant k for every c in [ a, b ]. Velocity vector may jump in size, but its direction is continuous. –G 2 : P’ ( c- ) = k P’ ( c+ ) for some constant k and P’’ ( c- ) = m P’’ ( c+ ) for some constants k and m for every c in [ a, b ]. Both 1 st and 2 nd derivative directions are continuous. Note that, for these definitions, G k continuity implies G i continuity for i < k. source: Hill, Ch 10 These definitions suffice for that textbook’s treatment, but there is more to the story…

Reparameterization Relationship Curve has G r continuity if an arc-length reparameterization exists after which it has C r continuity. “Two curve segments are G k geometric continuous at the joining point if and only if there exist two parameterizations, one for each curve segment, such that all i th derivatives,, computed with these new parameterizations agree at the joining point.” source: Farin, Ch 10 source: cs.mtu.edu

Continuity at Join Point Defined using parametric differential properties of curve or surface C k more restrictive than G k source: Mortenson Ch 3, p Defined using intrinsic differential properties of curve or surface (e.g. unit tangent vector, curvature), independent of parameterization. G 1 : common tangent line G 2 : same curvature, requiring conditions from Hill (Ch 10) & (see differential geometry slides) –Osculating planes coincide or –Binormals are collinear. Parametric Continuity Geometric Continuity

Parametric Cross-Plot source: Farin, Ch 6 For Farin’s discussion of C 1 continuity at join point, cross-plot notion is useful.

Composite Cubic Bezier Curves (continued) source: Farin, Ch 5 curves are identical in x, y space Parametric C 1 continuity, with parametric domains considered, requires (for x and y components): (5.30) Domain violates (5.30) for y component. Domain satisfies (5.30) for y component.

Composite Bezier Curves source: Mortenson, Ch 4, p For G 2 continuity at join point in cubic case, 5 vertices must be coplanar. (follow-up from prior slide) Achieving this might require adding control points (degree elevation). curvature at endpoints of curve segment consistent with:

C 2 Continuity at Curve Join Point “Full” C 2 continuity at join point requires: –Same radius of curvature* –Same osculating plane* –These conditions hold for curves p( u ) and r( u ) if: source: Mortenson, Ch 12 * see later slides on topics in differential geometry

Piecewise Cubic B-Spline Curve Smoothness at Joint source: Mortenson, Ch 5 curvature discontinuity familiar situation Incorrect: should be C 2

Control Point Multiplicity Effect on Uniform Cubic B-Spline Joint C 2 and G 2 control point multiplicities = 1 C 2 and G 1 One control point multiplicity = 2 C 2 and G 0 One control point multiplicity = 3 C 2 and G 0 One control point multiplicity = 4 One curve segment degenerates into a single point. Other curve segment is a straight line. First derivatives at join point are equal but vanish. Second derivatives at join point are equal but vanish.

If a knot has multiplicity r, then the B- spline curve of degree n has smoothness C n-r at that knot. Knot Multiplicity Effect on Non- uniform B-Spline source: Farin, Ch 8

A Few Differential Geometry Topics Related to Continuity

Local Curve Topics Principal Vectors –Tangent –Normal –Binormal Osculating Plane and Circle Frenet Frame Curvature Torsion Revisiting the Definition of Geometric Continuity source: Ch 12 Mortenson

Intrinsic Definition (adapted from earlier lecture) No reliance on external frame of reference Requires 2 equations as functions of arc length* s : 1)Curvature: 2)Torsion: For plane curves, alternatively: source: Mortenson *length measured along the curve Torsion (in 3D) measures how much curve deviates from a plane curve. Treated in more detail in Chapter 12 of Mortenson and Chapter 10 of Farin.

Calculating Arc Length Approximation: For parametric interval u 1 to u 2, subdivide curve segment into n equal pieces. source: Mortenson, p. 401 where using is more accurate.

Tangent unit tangent vector: source: Mortenson, p. 388

Normal Plane Plane through p i perpendicular to t i source: Mortenson, p

Principal Normal Vector and Line Moving slightly along curve in neighborhood of p i causes tangent vector to move in direction specified by: source: Mortenson, p Principal normal vector is on intersection of normal plane with (osculating) plane shown in (a). Use dot product to find projection of onto Binormal vector lies in normal plane.

Osculating Plane Limiting position of plane defined by p i and two neighboring points p j and p h on the curve as these neighboring points independently approach p i. Note: p i, p j and p h cannot be collinear. Tangent vector lies in osculating plane. i i source: Mortenson, p Normal vector lies in osculating plane.

Frenet Frame Rectifying plane at p i is the plane through p i and perpendicular to the principal normal n i : i i i source: Mortenson, p Osculating Plane Normal Plane Note changes to Mortenson’s figure 12.5.

Curvature Radius of curvature is  i and curvature at point p i on a curve is: source: Mortenson, p Curvature of a planar curve in x, y plane: Recall that vector lies in the osculating plane. Curvature is intrinsic and does not change with a change of parameterization.

Torsion Torsion at p i is limit of ratio of angle between binormal at p i and binormal at neighboring point p h to arc-length of curve between p h and p i, as p h approaches p i along the curve. source: Mortenson, p Torsion is intrinsic and does not change with a change of parameterization.

Reparameterization Relationship Curve has G r continuity if an arc-length reparameterization exists after which it has C r continuity. This is equivalent to these 2 conditions: –C r-2 continuity of curvature –C r-3 continuity of torsion source: Farin, Ch 10, p.189 & Ch 11, p. 200 Local properties torsion and curvature are intrinsic and uniquely determine a curve.

Local Surface Topics Fundamental Forms Tangent Plane Principal Curvature Osculating Paraboloid source: Ch 12 Mortenson

Local Properties of a Surface Fundamental Forms Given parametric surface p( u, w ) Form I : Form II : Useful for calculating arc length of a curve on a surface, surface area, curvature, etc. Local properties first and second fundamental forms are intrinsic and uniquely determine a surface. source: Mortenson, p

Local Properties of a Surface Tangent Plane source: Mortenson, p. 406 q p(ui,wi)p(ui,wi) components of parametric tangent vectors p u ( u i, w i ) and p w ( u i,w i )

Local Properties of a Surface Principal Normal Curvature Derive curvature of all parametric curves C on parametric surface S passing through point p with same tangent line l at p. source: Mortenson, p in tangent plane with parametric direction dw/du contains l normal curvature vector k n = projection of curvature vector k onto n at p normal curvature:

Local Properties of a Surface Principal Normal Curvature (continued) source: Mortenson, p typographical error? yes Rotating a plane around the normal can change the curvature  n. curvature extrema: principal normal curvatures

Local Properties of a Surface Osculating Paraboloid source: Mortenson, p. 412 Second fundamental form helps to measure distance of surface from tangent plane. As q approaches p: Osculating Paraboloid

Local Properties of a Surface Local Surface Characterization Elliptic Point: locally convex Hyperbolic Point: “saddle point” source: Mortenson, p typographical error? no Planar Point (not shown) Parabolic Point: single line in tangent plane along which d =0